Mechanical Properties Of Steel Fiber High-strength Self-compacting Concrete Under Freeze-thaw Cycles-sulfate Attack

Self-compacting concrete（SCC）has been widely used in complex structures and major projects due to its excellent plasticity,no noise and easy construction.With the increasing demand for the bearing capacity of concrete structures in tunnels,oceans,large spans,super high-rise buildings and other projects,the development and application of high-strength and ultra-high-strength SCC has attracted more and more attention.The toughening and crack resistance of steel fiber can effectively alleviate the high brittleness problem caused by the improvement of concrete strength,and the building structure often needs to be used in extreme environments such as low temperature,seawater,strong acid and strong alkali.It is necessary to consider the influence of these extreme environments to ensure that the building structure can maintain its stability and safety in long-term use.Therefore,in order to promote the application of high-strength SCC in key projects and ensure the structural safety during service,it is necessary to systematically study the performance evolution mechanism in extreme environments.In this paper,the basic mechanical properties,flexural properties and fracture mechanical properties of steel fiber high-strength SCC and the resistance to freeze-thaw cycle-sulfate attack with the volume fraction of steel fiber（0,0.30%,0.60%,0.90%and1.20%）and the number of freeze-thaw cycles-sulfate attack（0 times,150 times and 300times）were clarified through experimental research.The mechanism of freeze-thaw cycle-sulfate attack was clarified by the microscopic analysis,and the following conclusions were drawn:（1）Study on the physical and mechanical properties of steel fiber high-strength SCC:Steel fiber significantly improves the mechanical properties of high-strength SCC,but it is unfavorable to its workability.The 120 d mechanical properties of steel fiber high-strength SCC increase with the increase of steel fiber volume fraction.When the volume fraction of steel fiber is 1.20%,the compressive strength,flexural strength and splitting tensile strength reach the maximum values of 124.60 MPa,12.5 MPa and 8.2 MPa,respectively.Compared with the specimens without steel fiber,the initial flexural toughness ratio R_e,pand residual flexural toughness ratio R_e,500are increased by 56.25%and 12.20%respectively.（2）The evolution mechanism of structure and properties of steel fiber high-strength SCC under freeze-thaw cycles-sulfate attack:As the freeze-thaw cycles-sulfate attack times are increased,the mass loss rate of the high strength steel fiber SCC increases linearly,and the relative dynamic elastic modulus is decreased.After 300 times of erosion,the compressive strength,flexural strength and splitting tensile strength decreased by 7.12%,9.60%and 4.93%respectively.When the volume fraction of steel fiber is constant,the flexural toughness of steel fiber high-strength SCC decreases with the increase of freeze-thaw cycle-sulfate attack times.Under the influence of freeze-thaw cycle-sulfate attack,the pores and microcracks in concrete gradually increase under the expansion caused by the crystallization of sulfate attack products ettringite and dihydrate gypsum,and connect with each other,and the physical and mechanical properties of high-strength SCC deteriorate accordingly.（3）Study on the fracture mechanical properties of steel fiber high-strength SCC before and after freeze-thaw cycle-sulfate erosion:Before erosion,the peak load,fracture toughness,fracture energy and peak opening displacement of steel fiber high-strength SCC increased gradually with the increase of steel fiber volume fraction.Compared with the fracture specimen without steel fiber,the fracture energy increased by 19 times when the volume fraction of steel fiber was 1.20%.After 300 cycles of freeze-thaw cycles-sulfate attack,when the volume fraction of steel fiber was 1.20%,the peak load,fracture toughness and fracture energy decreased by 27.20%,25.39%and 31.79%,respectively,but the peak opening displacement increased by 5.43 times.